The present invention relates to the art of magnetic resonance cine imaging. It finds particular application in conjunction with quantitative flow cine imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with cardiac, angiography, circulatory, black blood cine, and other examinations in which flowing fluid is imaged.
Cine images have commonly been acquired using field echoes. Field echoes permit a rapid repetition rate, e.g. 64 sequence repetitions in 600 msec. In cardiac imaging, this enables a view of data for each of 64-100 frames or images to be collected in a single cardiac cycle. Each frame is displaced by about 10 milliseconds. When displayed sequentially, the frame images illustrate the cardiac cycle with a resolution of about 10 msec.
Generally, optimal cardiac gated cine images are achieved if all the views are collected within a single breath hold. Collecting all the views within a single breath hold eliminates the motion artifacts attributable to pulmonary motion. In the cardiac region, misrepresentation of heart position between breath holds can be significant.
A single breath hold is typically 16-20 heartbeats. Acquiring only one view per frame image in each cardiac cycle of a single breath hold would limit each frame image to 16-20 views. To increase the number of views per frame, data acquisition could be continued for several breath holds. However, the heart and surrounding tissue move with each pulmonary cycle and typically do not return accurately to the same position in subsequent breath holds.
One technique described in "Cineangiography of the Heart in a Single Breath Hold with a Segmented TurboFLASH Sequence", Radiology, Vol. 178, pp. 357-360, Atkinson and Edelman (RSNA, 1991), 128 lines or views of k-space were grouped into 8 segments of 16 views each. The resultant magnetic resonance echoes of each cardiac cycle were grouped into 16 groups of 8 views each. The number of groups are dependent on the patient's heart rate. The 8 views within each group of 16-20 consecutive heart beats were differently phase encoded and processed as 128 views (16.times.8) of the same frame image. In this manner, multiframe images of the cardiac cycle, each with 128 views per image, were generated. More specifically, in the first heart beat, views or lines 1, 17, 33, etc. were collected; in the second heart beat, lines 2, 18, 34, etc. were collected; and so forth. The raw data from these sets was combined or interleaved to form the 128 view data set for reconstruction into each of the multiple frames.
One of the drawbacks of the Atkinson and Edelman segmentation of k-space was that the resultant frames suffered from blurring. The present inventor has recognized that this blurring is attributable to the acquisition of the central views, the views with the most signal power and image information, at different times in subsequent cardiac cycles. In the above-illustrated 8 segment, 16 heart beat sequence, the central views in each group alternated between two different time displaced points in the cardiac cycle.
A common problem in magnetic resonance scanners is that eddy currents are generated in the cryostat by rapidly changing magnetic gradients, e.g. the phase encode gradients. These eddy currents cause a number of different image artifacts and distortions. Commonly, the effects of eddy currents are minimized by using sequences that have gradients of alternating polarity such that the eddy currents tend to cancel. One problem with the Atkinson technique is that it acquired views from both positive and negative k-space in a single heartbeat or repetition. If there is a rewind pulse, this resulted in two phase encode gradients of the same polarity being applied back-to-back. The back-to-back applications of two gradients of the same polarity tended to cause the gradient currents to add, rather than cancel, increasing gradient current artifacts and distortions.
In order to push DC artifacts and un-encoded signals from the center to the edge of an image, it is common to perform 0.degree.-180.degree. phase cycling of the RF. For example, the phase of the RF pulse is offset 180.degree. in alternate views and cycles. With the Atkinson and Edelman technique, with some group and segment sizes, each group had both even and odd numbered views with even numbered views or odd numbered views back-to-back. This made 0.degree.-180.degree. phase cycling impossible.
In accordance with the present invention, a symmetric, centrally-ordered phase encode grouping of the views is provided which overcomes the above-referenced problems and others. The present invention further provides for a quantitative flow imaging technique which uses the symmetric, centrally-ordered phase encode grouping, as well as other segmentations of k-space.